1. Field of Invention
This invention relates to indirect evaporative cooling technology, and particularly to heat exchangers useful in indirect evaporative cooling devices used for conditioning air.
2. Description of Related Art
Evaporative cooling involves lowering the temperature of a liquid by utilizing the latent heat of vaporization of a portion of the liquid. The term “Indirect Evaporative Cooling” was coined by personnel at Des Champs Laboratories in 1974, when they decided to enhance summer-time air-to-air energy recovery, from building exhaust air, by utilizing the wet bulb temperature of the exhaust air instead of the higher dry bulb temperature. At the time, it was common practice during summer months to transfer energy from the cooler exhaust air to the warm, outdoor, make-up air by using an air-to-air heat exchanger. The driving force that causes the transfer of energy within the heat exchanger, in the aforementioned process, is the sensible temperature difference between the two air streams. During summer months, the outdoor air that is delivered to a space, and the recirculated internal air, are usually air-conditioned. As a result, the air within the space has a lower wet bulb temperature than the outdoor air or the inside dry bulb temperature.
By spraying water on the surface of the exhaust side of the air-to-air heat exchanger during the cooling season, the exhaust air flow, at a low wet bulb temperature, evaporates water from that exhaust side surface and thereby attempts to drive the water/exhaust-side surface temperature lower, approaching the exhaust air wet-bulb temperature at the limit. The supply air, flowing on the other side of the membrane that separates the two air streams, comes in contact with a surface (the opposite side of the membrane from the exhaust side) that is much cooler and consequently more energy is transferred between air streams and thus a greater energy saving occurs. The reason the surface is cooler than it would otherwise be is because of the evaporative cooling that takes place at the exhaust air/water layer interface, which in turn manifests itself as a cooler membrane temperature than would exist if the exhaust air were simply left dry with no water spray. As a matter of interest is the fact that the temperature drop across the membrane, from the exhaust-side surface to the supply-side surface, is very small, i.e., on the order of a fraction of a degree while the typical temperature difference between the two bulk air streams is on the order of 10 to 40° F.
Early indirect evaporative cooling (IEC) units were simply a modification of standard air-to-air heat exchangers that were used to extract energy (or lack of energy) from the exhaust air and transfer it to fresh, incoming make-up air, thus reducing the energy that would otherwise be required to condition the outdoor air prior to delivering it to the occupied space. Consequently, the heat transfer devices used in the early IEC units were designed to transfer energy in a dry environment. In contrast, more recent IEC units are subjected to a wet environment. Such wet environments are known to contain a wide range of contaminants and are often corrosive to IEC components. As a result of the hostile environment that such IEC heat exchangers witnessed, they were maintenance prone and short lived. Consequently, IECs, after getting off to an admirable start in the late 1970s and early 1980s, languished in the 1990s and, so far, into the new century even though IECs have the potential for tremendous energy savings and reduction in peak summer electrical demand.
Additionally, known heat exchangers have designs that require lengthy assembly periods. For example, in known systems, assembly of heat exchanger tubes to a plate or manifold requires an individual to seal around the perimeter of each tube by hand in an attempt to prevent leaks. This method of assembly often requires 10–20 hours to implement. Furthermore, extensive quality assurance is also necessary due to the possibility of leaks.